Field of the Invention
The present invention relates to a current-to-voltage conversion circuit, a reception apparatus, and an imaging system.
Description of the Related Art
In a current-output type sensor configured to output a current, an alternating current (AC) component of an output signal may be an effective signal, and a direct current (DC) component thereof may be disturbance. This DC component may be an unnecessary component that narrows an operation range of a circuit when the output signal of the current-output type sensor is processed. Therefore, a method of removing the DC component and passing the AC component is necessary.
As the method of removing the DC component, a method using a coupling capacitor is known. This method can block the transmission of the DC component, and can pass the AC component by converting the current output from the sensor to a voltage, and then transmitting the voltage to a subsequent stage via the coupling capacitor. This method can accurately remove the DC component when the DC current contained in the output from the sensor is sufficiently smaller than the AC current.
However, when the DC current is relatively larger than the AC current, in order to prevent a circuit operation saturation when converting the current to the voltage, a nonlinear voltage conversion such as a logarithmic compression may be carried out. In this case, the signal-to-noise ratio (SNR) may degrade. In other words, when an extent of the change in the current (dynamic range) of the DC component or the AC component of the output signal is large due to an external factor such as an operation environment of the sensor and a variation in the output of the sensor, this method has a limit.
As a solution for this problem, in Japanese Patent Application Laid-Open No. 2010-249786, the DC component and the AC component are separated while the sensor signal is still the current signal, and only the AC current is finally processed as an effective signal by an operational amplifier. This method uses a feedback circuit so as to extract only the DC component of the sensor signal, and subtract a replica current having the same magnitude as the DC component from the sensor signal current, thereby acquiring the AC current. An extraction accuracy of the DC component increases as the DC current increases with respect to the AC current, and hence this method can prevent the SNR from degrading.
In the technology according to Japanese Patent Application Laid-Open No. 2010-249786, a circuit for a signal processing unit configured to detect the DC component of the sensor output so as to generate the replica current, and a circuit for a unit configured to calculate the AC component partially overlap each other. The DC component and the AC component are separated depending on a frequency characteristic of an impedance existing in a sensor output node, and this frequency characteristic is determined by a circuit constant. The existence of the overlapping portion between the circuits generates a restriction on a setting range of the frequency characteristic for separating the DC component and the AC component from each other. As a result, the signal separation performance for the DC component and the AC component may be insufficient, and simultaneous satisfaction of the removal of the DC component in a wider dynamic range and the detection of the AC component in a wide dynamic range is limited.